This disclosure generally relates to blower systems (also referred to as air handler systems) and other fluid handling systems, and more particularly to controls for such systems.
Heating, ventilation and/or air conditioning (“HVAC”) systems commonly have blower systems for moving air. These blower systems typically include a fan (such as a squirrel cage fan), an electric motor for powering the fan, and a control for the electric motor. In some systems, the control receives a signal corresponding to airflow demand from a system controller, such as a thermostat. The control determines a motor torque needed to meet the airflow demand and sends a signal to the motor to provide the needed torque so the fan produces the demanded airflow. Therefore, to accurately provide the demanded airflow, the control must accurately determine the torque demand needed to provide the airflow demand.
A variety of methods is used to characterize a system so that airflow demands can be converted to torque demands. For example, U.S. Pat. Appn. Pub. No. 2007/0248467 A1 describes a method for producing a torque demand from an airflow demand using an equation such as:T=K1+K2*s+K3*A+K4*s*A2,where T represents the torque demand in Newton meters (Nm), s represents motor speed in revolutions per minute (rpm) and A represents the airflow demand in cubic feet per minute (cfm). K1, K2, K3 and K4 are constants derived for a particular blower system. These constants are derived from torque, speed and airflow data collected for the particular blower system. Other equations (e.g., higher order equations) may be used to calculate airflow demands from torque demands. Regardless of the particular equation used, constants must generally be determined for each system because the constants vary with various system parameters such as fan type, surrounding pressure, and surrounding temperature, as well as downstream parameters such as duct configuration, filter type, and filter condition. The process of determining constants for a system is referred to as system characterization.
System characterization is usually performed in a calibrated test chamber, measuring airflow, blower speed and torque at different static pressures representing different downstream conditions. Characterization is generally a lengthy process and each system must be separately characterized. Systems are difficult to characterize when installing motors in the field such as when replacing a used motor to repair a blower system. Thus, there is a need for a method of characterizing systems in the field without using a calibrated test chamber that minimizes prediction errors when converting an airflow demand (or another system demand) into a torque demand.